The demand for less expensive, and yet more reliable integrated circuit components for use in mobile communication, imaging and high-quality video applications continues to increase rapidly. As a result, integrated circuit manufacturers are requiring greater accuracy in voltage references for such components and devices to meet the design requirements of such myriad emerging applications.
Voltage references are generally required to provide a substantially constant output voltage despite gradual or momentary changes in input voltage, output current or temperature. In particular, many designers have utilized bandgap reference circuits due to their ability to provide a stable voltage supply that is insensitive to temperature variations over a wide temperature range. These bandgap references rely on certain temperature-dependant characteristics of the base-emitter voltage, VBE, of a transistor. Typically, these bandgap reference circuits operate on the principle of compensating the negative temperature coefficient of a base-emitter voltage, Vbe, of a bipolar transistor with the positive temperature coefficient of the thermal voltage, i.e., with VThermal=kT/q, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electronic charge. In general, the negative temperature coefficient of the base-emitter voltage VBE is summed with the positive temperature coefficient of the thermal voltage VThermal, which is appropriately scaled such that the resultant summation provides a zero temperature coefficient.
Conventional bandgap technologies generally comprise circuits designed to generate a positive temperature coefficient through a proportional-to-absolute-current IPTAT flowing through a resistor. For example, with reference to FIG. 1, a bandgap circuit 100 configured to provide a bandgap voltage VBG of approximately 1.2 volts comprises a positive temperature coefficient generated by a proportional-to-absolute-current IPTAT flowing through a resistor R, and a negative temperature coefficient of the base-emitter voltage VBE generated from a bipolar transistor Q1. Proportional-to-absolute-current IPTAT is also typically generated by another bipolar and resistor circuit.
As the available quiescent current is reduced in bandgap circuit 100, the size of resistor R, as well as the size resistor used to generate proportional-to-absolute-current IPTAT, must be suitably increased to obtain the necessary positive temperature coefficient to counterbalance the negative temperature coefficient. For example, to maintain a positive temperature coefficient voltage (IR) drop of approximately 0.6 volts, if a bias current is reduced to 50 nA, then at least a 12 Mohm value resistor R is required to maintain the necessary IR drop, as well as a smaller resistor, e.g., approximately 360 Kohm to 1 Mohm depending on emitter ratio, used to generate proportional-to-absolute-current IPTAT. Integrated resistors of this size are not practical due to space limitations.